This project entails the use of photochemical crosslinking techniques to investiate the structure of (a) the binding sites of ribosomal proteins on E. coli ribosomal RNA and (b) the peptidyl- and aminoacyl-tRNA binding sites on E. coli ribosomes. Specially developed aryl azide reagents are attached non-photochemically to available thio or amino groups in selected ribosomal proteins and tRNA molecules. The modified components are then allowed to form specific complexes with the ribosomal RNA or the 70S ribosome, as appropriate. Irradiation results in the photochemical decomposition of the azides to highly reactive nitrenes which can insert into neighboring molecules of the complex. The sites of insertion in ribosomal RNA will be determined by nucleic acid sequencing methods. Crosslinks to proteins will be investigated by chromatography and by one- and two-dimensional gel electrophoresis. The results of these experiments should aid in defining the spatial arrangement of constituents within functional macromolecular aggregates.